1. Field of the Invention
The present invention relates to a binder resin composition, a curing agent composition, a caking additive composition and a casting mold composition for a self-hardening or gas-hardening mold process, a sand mold for casting and a process for producing the same. More particularly, the present invention relates to a binder resin composition, a curing agent composition, a caking additive composition and a casting mold composition which are useful in a process for the production of a sand mold for casting by molding a refractory particulate material with a water-soluble phenolic resin as a binder and an organic ester as a curing agent and which can remarkably improve the reusability of the refractory particulate material. The present invention also contemplates a sand mold for casting produced by using one of the above-mentioned compositions and a process for the production of a sand mold for casting comprising the use of one of the above-mentioned compositions.
2. Description of the Related Art
The self-hardening molding process, the Cold Box process and the Croning process (i.e., the shell process) have been known as processes for producing a mold, such as a master mold or core, by the use of an organic binder. Particularly from the standpoint of productivity, quality of castings, safety, and hygiene, the self-hardening or self-curing molding process using an organic binder instead of an inorganic binder has already become a general-purpose molding process, mainly in the field of machinery castings.
Meanwhile, the Croning process of thermally curing a particulate refractory coated with a phenolic resin, i.e., a so-called coated sand, has been also widely used as a process for producing a mold at a medium or high rate. However, the employment of the Cold Box process of conducting the curing or hardening of a mold with a gas or an aerosol at ordinary temperatures, instead of the Croning process has been earnest attempted in order to save the energy that would otherwise be consumed in the production of the mold, and improve the production rate of the mold and the qualities of the mold and the casting.
Recently, compositions for a molding sand, comprising a water-soluble phenolic resin as a binder and an organic ester as a curing agent, have been reported in U.S. RE Nos. 32812 (published on Dec. 27, 1988; Borden UK Limited) and 32720 (published on Jul. 26, 1988; Borden UK Limited) as compositions which are suitably used in the self-hardening or gas-hardening molding process, and which can improve the quality of the casting and the working atmosphere. The molding process using such a composition is advantageous in that the working atmosphere is not polluted by the sulfur dioxide gas evolved during pouring, and in that the obtained casting has few faults due to sulfur and nitrogen, because the composition does not contain sulfur or nitrogen, unlike the acid-curing type binder composition of the prior art. However, It is well known that the employment of the above-mentioned process, i.e., a process using such a composition comprising a water-soluble phenolic resin and an organic ester, is limited due to the low reclaimability of molding sand. Accordingly, the improvement of the process in these respects has been much desired. Further, the amount of the water-soluble phenolic resin, or the composition comprising the same to be used, must be increased in order to attain the necessary mold strength because the obtained mold is poor in strength when such a composition is employed. Additionally, the use of a composition comprising a water-soluble phenolic resin and an organic ester has the disadvantage that an increasing amount of the resin must be added to the sand when the amount of sand recovered for reuse or reclaimed after repeated use is large, because the production of a mold having a desirable strength becomes more difficult when the sand recovered for reuse or reclaimed after repeated use is employed. What is worse, such an increase in the resin content of a mold leads to an increase in the amount of thermal decomposition gas evolved in pouring, which introduces gas faults into the casting, and adversely affects the working atmosphere.
In order to alleviate these disadvantages even a little, for example, methods such as subjecting the used sand to severe mechanical abrasion to remove the organic substance and/or alkali remaining on the surface of the sand in preparing the sand for reuse, increasing the amount of fresh sand used, or using sand only once and then throwing it away are now generally employed. Accordingly, the rate of reclamation of sand, i.e., the rate of the used sand based on the entire sand, in the case of using the above-mentioned composition was at most about 85% ("Modern sand binders--a review; Liquid catalyst cold setting processes, Liquid ester cured alkaline phenolic binders ALPHASET", Foundry Trade Journal, 8/22, P.924, December 1989, by BRITISH CASTING IRON RESEARCH ASSOCIATION).
This disadvantage specialty in sand reclaimability in the case of using a composition comprising a water-soluble phenolic resin and an organic ester is made more apparent by comparison to the use of an acid-curing furan resin binder, which is generally used as a binder for molding the sand. When an acid-curing furan resin binder is used, the mold strength attained by using reclaimed sand is generally higher than that attained by using fresh sand, so that the amount of the binder to be added to reclaimed sand can be slightly reduced as compared with that to be added to fresh sand. Further, severe mechanical abrasion is unnecessary for the reclamation of sand, so that the rate of reclamation of sand is about 95% or above.
In the production of a mold and/or a core from the sand recovered for reuse, in which sand is bound with a binder after curing, the method for reclamation of sand and the rate of reclamation of sand are economically important factors.
The reclamation of sand from a mold and/or a core is generally conducted by taking a casting from the mold, breaking the mold and the core into pieces by mechanical oscillation or similar treatment for breaking up the mold and the core, crushing the lumps and agglomerate, and recovering the sand. The recovered sand is generally subjected to a reclamation treatment in order to remove unburned binder remaining on the surface of the sand. There are three known processes for reclaiming sand (i.e., the mechanical, wet and thermal processes).
The wet reclamation process is relatively unfavorable because of the problematic disposal of washings, and the high energy cost of drying the sand. Further, the thermal reclamation process is also relatively unfavorable because of its high energy cost. The mechanical reclamation process is most economical, so that it is most generally employed; its use is becoming more widespread in the casting industry.
The use of sand thus reclaimed, however, has the aforementioned disadvantage inherent in this process, that a binder system comprising a step of curing a water-soluble phenolic resin with an organic ester cannot provide a mold having a sufficiently high strength. This phenomenon is far different from that which occurs using an acid-curing furan resin. Accordingly, an improvement of the above-mentioned disadvantage has been earnestly desired.
As processes for improving the strength of a mold produced by the use for reclaimed sand, a process of lowering the content of solid, i.e., resin, contained in the binder composition is disclosed in Japanese Patent Laid-Open No. 262042/1989 (Oct. 18, 1989) and the corresponding European Patent Publication-A No. 336534 (Oct. 11, 1989). A process for preliminarily treating reclaimed sand with a silane solution is disclosed in Japanese Patent Laid-Open No. 262043/1989 (Oct. 18, 1989) and the corresponding European Patent Publication-A No. 336533 (Oct. 11, 1989). However, these processes do not impart a satisfactory strength to molds made from reclaimed sand, though they sometimes slightly enhance the strength of such a molds.